A carbonylation process of a methanol method (an acetic acid production process of a methanol method) is known as an industrial method for producing acetic acid. In this process, for example, methanol and carbon monoxide are reacted in the presence of a catalyst in a reaction vessel to produce acetic acid. The reaction mixture is evaporated in an evaporator, and the vapor phase is purified in a lower boiling point component removal column and subsequently in a dehydration column so that product acetic acid is prepared. Alternatively, product acetic acid is prepared via a higher boiling point component removal column subsequent to the dehydration column, and further, a product column.
In such a process for producing acetic acid acetaldehyde formed through reduction of methyl iodide is changed to crotonaldehyde through an aldol condensation to deteriorate a potassium permanganate test value (permanganate time) of the product acetic acid. Further, crotonaldehyde is changed to 2-ethyl crotonaldehyde through an aldol condensation with acetaldehyde, and 2-ethyl crotonaldehyde also deteriorates a potassium permanganate test value of the product acetic acid. However, the degree of deterioration of a potassium permanganate test value per unit mass by crotonaldehyde is severer than the same by 2-ethyl crotonaldehyde, and quality deterioration becomes more significant, when crotonaldehyde is contained in a product acetic acid.
For reducing crotonaldehyde or 2-ethyl crotonaldehyde, there have been heretofore two major groups of industrially applied methods, namely:
(i) a method for suppressing formation of crotonaldehyde in a reaction system by means of separation and removal of acetaldehyde by-produced in a reaction system from methyl iodide in a purification step so as to decrease acetaldehyde in methyl iodide recycled to the reaction system; and(ii) a method by which crotonaldehyde contained in crude acetic acid obtained in the course of a purification step is directly decomposed by oxidation with ozone (Patent Literatures 1 and 2). However, a separation and removal apparatus for acetaldehyde and an ozonization apparatus are both expensive. Improvement of a potassium permanganate test value of product acetic acid has heretofore relied entirely on the methods, which leads to increase in apparatus costs.
Meanwhile, it has been known that alkanes are formed as impurities in an acetic acid production process of a methanol method. The alkanes have a carbon number of not less than 3 and are impurities with a boiling point higher than that of methyl iodide or methyl acetate. They are mainly saturated or unsaturated hydrocarbons, or occasionally contain an oxygen atom or an iodine atom in the molecule. Japan Patent Laid Open No. 04-295445 discloses a technology, by which for removing the alkanes an organic phase of the column top condensate of a lower boiling point component removal column is distilled in a distillation column (alkane removal column) to recycle the column top distillate liquid containing methyl iodide, methyl acetate, and a carbonyl impurity to a reactor, or feed the same to an acetaldehyde removal column, and water is added to the bottom fraction of the column bottom containing alkanes, water, and acetic acid for extraction to recycle the aqueous phase containing acetic acid to a reactor, and to discard the organic phase containing alkanes. However, the literature neither discloses nor suggests how to improve a potassium permanganate test value of the product acetic acid.